1. Field of the Invention
The invention relates to a multi-layered circuit board. More particularly, the invention relates to a multi-layered circuit board with an electro-static discharge (ESD) protection structure.
2. Description of Related Art
During or after fabrication of an integrated circuit (IC), the IC is often damaged due to electrostatic discharge (ESD). For instance, the ESD frequently harms an electronic device (e.g., a chip) assembled onto a printed circuit board (PCB). Hence, the IC itself is required to have an ESD protection function, so as to prevent the external electro-static current from impairing the IC. In addition, during production and assembly of a PCB, the PCB is frequently in contact with hands of workers, other PCBs, assembly machine or testing tools. Hence, the ESD is likely to cause damages to the PCB itself or the IC on the PCB.
In most cases, the ESD protection of the electronic device refers to prevention of accumulation of electro-static charges, so as to avoid instant discharge resulting from the accumulated electro-static charges. To preclude the accumulation of electro-static charges, the electro-static charges are often released through point discharge (or marginal discharges). When the electro-static charges are accumulated in a conductor, the charges are distributed on an outer surface of the conductor because the net electric field within the conductor is zero. The outer surface of the charged conductor in electro-static equilibrium is an equipotential surface. Besides, the larger the surface radius of curvature of an object, the higher the density of charges; the smaller the surface radius of curvature of an object, the lower the density of charges. When the surface radius of curvature of an object is large (e.g., at a tip), the electric field increases drastically such that air around the object is ionized and converted into positive and negative ions. Discharge then occurs at the tip which attracts and neutralizes counter ions, which is the so-called point discharge (or marginal discharges).
Another way to take precautions to avoid the ESD from impairing an electronic device refers to improvement of ESD protection capability of the electronic device itself (e.g., IC). For instance, resistors connected in series or in parallel may be formed on the circuit to restrain the amount of ESD current that flows through the IC, so as to effectively prevent the ESD from impairing the electronic device. In case that the ESD is devastating, it is likely to use transient voltage suppressors (TVS) or Zener diodes connected in parallel for current restraint. However, the use of TVS or Zener diode not only complicates the circuit design but also increases the manufacturing costs. Moreover, the use of TVS or Zener diode is against the trend of miniaturizing the dimensions of circuit boards.